The present invention relates to contamination testers, and more particularly, to an apparatus for non-destructive inspection of blind holes.
In order to achieve maximum efficiency of performance in high technology products--such as airfoils, engines, etc.--special non-destructive inspection (NDI) discipline has developed. To date, NDI tools have been developed which are very sensitive to surface contamination. These tools include surface potential difference, photoelectron emission and ellipsometric testers.
The problem is to develop NDI tools which can perform in even the closest of areas. This arsenal should be one that can inspect all areas of surface-treated panels or parts. The need for sophisticated equipment that can scan curved or shaped parts is characterized by the critical nature of and the great number of parts to be inspected.
The solution to the problem lies in the development of NDI tools that can detect deviations from an acceptable surface condition norm. The first step involves characterization of an acceptable surface, i.e., identification of the boundaries of signals from various surface tools for which the surface is acceptable. The second step involves establishing which tools can detect which deviations from the acceptable surface. The third step involves scanning of surfaces with the surface tools to identify regions that deviate from the acceptance band (i.e., that are contaminated).
Ellipsometry is one NDI reflective technique wherein the sample is not touched by the instrument. In one embodiment, a beam of polarized monochromatic light is reflected from the surface. Although the incident beam is plane polarized at an azimuth of 45.degree. with respect to the plane of incidences (POI), the reflected beam is elliptically polarized. The parameters measured by the ellipsometer are the phase shift .DELTA. of light polarized perpendicular to the POI with respect to that polarized parallel to the POI and .psi., the arctangent of the reflection coefficients for these components. An advantage of the ellipsometer is that .DELTA. and .psi. are absolute values that are not dependent on the absolute light intensities but only on the ratio of the intensities. Although .psi. is very sensitive to surface roughness, it is relatively insensitive to the thickness of dielectric (e.g., oxide) films on the surface. In contrast, .DELTA. is extremely sensitive to film thickness and relatively insensitive to surface roughness. The phase shift .DELTA. can be used to detect as little as 0.1 .lambda. to as much as 5000 .lambda.of oxide or hydroxide on a properly anodized aluminum surface or a film of contamination on top of the oxide with the same resolution.
The present invention is an adaptation of a hand-holdable contamination tester employing an off-null ellipsometric technique which is disclosed in my U.S. Pat. No. 4,381,151, and reference should be made thereto for further background information.
This device and other known ellipsometric devices fail, however, to be operable in regions such as small "blind holes". In many instances it is necessary to prepare surfaces for painting, adhesive bonding, electroplating, etc., in blind holes that are difficult or impossible to reach with normally used surface tools.
It is therefore an object of the present invention to provide an off-null ellipsometric device which is operable in hard to reach regions such as blind holes.